Acute Dermal Toxicity and Analgesic Effect of a Capsaicinoid Gel Against Formalin-Induced Pain in Wistar Rats

Abstract

Background: Pain is a major public health concern, often inadequately managed due to the limitations and side effects of conventional analgesics. Capsicum annuum, widely used in traditional medicine, contains capsaicin, a compound with known analgesic properties. Despite its therapeutic promise, data on the dermal toxicity and efficacy of capsaicin-based topical formulations remains limited. Objective: The main objective of this study is to evaluate the acute dermal toxicity and analgesic efficacy of a capsaicinoid gel derived from C. annuum in Wistar rats. Methods An acute dermal toxicity test was conducted in female Wistar rats using OECD guideline 402. Rats received a single dermal application of 5% capsaicinoid gel at 200, 1000, or 2000 mg/kg and were observed for clinical signs over 14 days. Analgesic activity was evaluated using a formalin-induced paw-licking model. Male rats received topical treatments of capsaicinoid gel (100, 200, or 400 mg/kg) or 1% diclofenac gel (positive control), followed by formalin injection. Pain behaviour was scored in early (0-10 minutes) and late (15-60 minutes) phases. Hematological and biochemical parameters were assessed after 21 days. Results: No mortality was observed. Transient dermal and autonomic effects (e.g., erythema, tremors) were noted at higher doses but resolved spontaneously. The gel significantly reduced formalin-induced pain behaviours in both test phases, particularly at higher doses. Hematological and biochemical parameters showed mild, dose-related changes but remained within physiological ranges. Conclusion: The 5% capsaicinoid gel exhibited promising analgesic effects and an acceptable safety profile in rats. These findings support its potential as a topical analgesic, warranting further formulation optimization and chronic toxicity evaluation.